Flat panel displays and other devices that exploit the principle of frustrated total internal reflection (FTIR) to induce the emission of light from the system may have to satisfy crucial physical criteria to function properly. The display system disclosed in U.S. Pat. No. 5,319,491, which is incorporated by reference in its entirety herein, as representative of a larger class of FTIR-based devices, illustrates the fundamental principles at play within an FTIR-based device. Such a device is able to selectively frustrate the light undergoing total internal reflection within a (generally) planar waveguide. When such frustration occurs, the region of frustration constitutes a pixel suited to external control. A rectangular array of such regions, which are often controlled by electrical/electronic means, is fabricated upon the top active surface of the planar waveguide. This aggregate structure, when suitably configured, functions as a video display capable of color generation usually by exploiting field sequential color and pulse width modulation techniques.
The criteria to be satisfied for FTIR systems to function properly involve two fundamental areas: the preconditions for frustration, and the preconditions for non-frustration. There are many mechanisms available to frustrate total internal reflection (five of which are articulated in U.S. Pat. No. 5,319,491), all of which lead to a pixel being in an “on state” (emitting light through the “window” dynamically created in the planar waveguide). At issue is the physical configuration to secure a suitable “off state” where light is intended to remain within the planar waveguide across a given pixel region.
The off state (quiescent, inactive state) of individual pixels on a display, and indeed of the display in general, is of the highest importance. If some light is always leaking (by spurious emission, frustration, or other cause) from the display (at the pixels, between the pixels, or in general), this constitutes system noise that compromises the quality of the signal. The contrast ratio of a display is based on its signal-to-noise ratio, and contrast ratio serves as a primary index of display quality and accuracy. Therefore, if an FTIR display emits noise (light when and where no light is supposed to be emitted), this harms the display's quality.
Noise arises when total internal reflection is frustrated when and where it should not be. Different causes can give rise to such system noise, and in most displays more than one cause is operative to add to the noise level. For example, the optical quality of the material selected for the planar waveguide has a direct bearing on noise. If the material has many scattering domains distributed through it (so that it becomes more translucent than transparent), some of the light scattered off these domains will be scattered at angles that do not conserve total internal reflection. For this reason, the waveguide will glow in proportion to the amount of scattering domains distributed within it, thereby raising the noise floor. The solution to this problem is to fabricate the waveguide from the most optically transmissive materials available, thereby securing a meaningful reduction of the noise floor with respect to this specific source of system noise.
Other noise sources within FTIR systems do not have so straightforward a solution route. The first involves errors in waveguide geometry (the limits of parallelism and orthogonality), while the second involves noise at the interface of the waveguide and any superadded cladding layers (which can serve to support various required pixel control mechanisms, protect the display surface from external trauma, and/or other purposes). These are sources of system noise (light leakage) that do not have a straightforward solution route.
Therefore, there is a need in the art for a means to reduce light leakage (system noise), and thus improve contrast ratio performance, in FTIR display devices where the leakage is due to geometric imperfections in waveguide fabrication and/or leakage at the interface of the waveguide to superadded cladding structures.